Technical Field
Embodiments of the subject matter disclosed herein generally relate to methods and systems and, more particularly, to mechanisms and techniques for monitoring a given geological structure in a subsurface in a quick and efficient manner.
Discussion of the Background
Land and marine seismic data acquisition and processing generate an image of a geophysical structure (subsurface). While this image/profile does not provide a precise location for natural resources, it suggests, to those trained in the field, the presence or absence of these resources. Thus, providing a high-resolution image of the subsurface is an ongoing process for the exploration of natural resources, including, among others, oil and/or gas.
During a marine seismic gathering process, as shown in FIG. 1, a vessel 110 tows an array of seismic receivers 111 located on streamers 112. The streamers may be disposed horizontally, i.e., lying at a constant depth relative to the ocean surface 114, or may have spatial arrangements other than horizontal, e.g., variable-depth arrangement. Vessel 110 also tows a seismic source array 116 configured to generate a seismic wave 118. Seismic wave 118 propagates downward, toward the seafloor 120, and penetrates the seafloor until, eventually, a reflecting structure 122 (reflector) reflects the seismic wave. The reflected seismic wave 124 propagates upward until it is detected by receiver 111 on streamer 112. Based on this seismic data, an image of the subsurface is generated. If a single streamer is used, the image is a 2-dimensional (2D) image. If plural streamers are towed by the vessel simultaneously, the image is 3D. A 4D image is obtained when two 3D surveys are performed for the same area with a given time interval, usually months or years.
Alternatively, ocean bottom cables (OBC) or ocean bottom nodes (OBN) and seismometers (OBS) may be used to record the seismic data. FIG. 2 shows an OBC 130 that includes plural receivers 132 distributed on the ocean bottom 120. The plural receivers 132 are connected to each other with a cable 133 that may also be connected to a data collection unit 134. Various means (e.g., underwater vehicle) may be used to retrieve the seismic data from the data collection unit 134 and bring it on the vessel 110 for processing. 3D images may be generated with such configuration.
For a land seismic survey, a system 300 for the acquisition of 4D seismic data includes plural receivers 312 (e.g., hydrophones, accelerometers, etc.) positioned over an area 312a of a subsurface to be explored and in contact with the surface 314 of the ground or in the ground. A number of seismic sources 316 (e.g., vibratory elements) are also placed on surface 314 in an area 316a in a vicinity of receivers 312. A recording device 318 is connected to the plurality of receivers 312 and placed, for example, in a station-truck 320. Each source 316 may be composed of a variable number of vibrators, typically between 1 and 5, and may include a local controller 322. A central controller 324 may be present to coordinate the shooting times of the sources 316. A GPS system 326 may be used to time-correlate the shooting of sources 316 and data acquisition by receivers 312.
With this configuration, sources 316 are controlled to generate seismic waves, and the plurality of receivers 312 records waves reflected by oil and/or gas reservoirs and other structures. The seismic survey may be repeated at various time intervals, e.g., months or years apart, to re-image the subsurface in order to determine changes in the reservoirs. Although repeatability of source and receiver locations is generally easier to achieve onshore, variations caused by changes in near-surface can be significantly larger than reservoir fluid displacement, making time-lapse 4D seismic acquisition and repeatability challenging.
One or more of the above-noted techniques may be used to monitor a producing reservoir. For these instances, the goal of 4D processing is to determine how and where earth properties change by evaluating differences in processed seismic data acquired at different times, usually before (i.e., the baseline survey) and after (i.e., the monitor survey) a period of fluid production from a petroleum reservoir. Success of 4D processing depends on the accuracy with which differences in acquisition or subsurface changes not related to fluid production are compensated for during data processing and imaging, in order that 4D noise (the difference of migrated images not related to fluid production) is kept reasonably quiet. Relevant sources of 4D noise include differences in wave field sampling caused by different survey acquisition parameters between baseline and monitor.
Currently, the 4D seismic solutions are aimed at providing an update of the full subsurface 3D structural image. In other words, both the base and monitor surveys collect a substantial amount of seismic data for generating a full subsurface 3D structural image of the volume of interest. A comparison of the full monitor survey with the previous full base survey is intended to provide the 4D changes induced by oil production.
However, a full 3D seismic survey acquires a large amount of seismic data, takes time (weeks if not months), lacks accurate repeatability and incurs a high cost. All these factors work against the 4D seismic survey, making this tool slow and expensive in spite of its usefulness.
Thus, there is a need to acquire a 4D seismic survey faster, cheaper and with more accuracy.